GABA-A receptors and Addiction - recommended reading (references)

Question 1

GABA-A Receptor; Positive and Negative Allosteric Modulators

Author

Answer 1

Olsen

Question 2

GABA-A Receptor; Positive and Negative Allosteric Modulators

Abstract

Answer 2

GABARs are members of the ligand-gated ion channel receptor superfamily, a chloride channel family

The subtypes present on GABARs demonstrate different brain regional and subcellular localization, age-dependent expression and potential for plastic changes with experience including drug exposure

GABARs are targets of agonist depressants and antagonist anticonvulsants, but most GABAR drugs act at an allosteric site

GA including alcohols and neurosteroids act at GABAR subunit-interface trans-membrane sites

Ethanol at high anaesthetic doses and low intoxication doses acts on GABAR subtype dependent transmembrane domain s

Question 3

GABA-A Receptor; Positive and Negative Allosteric Modulators

GABAA vs GABAB

Answer 3

Bicuculline is a specific GABAA antagonist
Baclofen is a specific GABAB agonist

Both types of GABA receptor participate in both postsynaptic inhibition and presynaptic inhibition. Both forms of GABA receptors exist at both synaptic and extrasynaptic locations

GABAArs demonstrate considerable extrasynaptic localization where they function to mediate the tonic inhibitory currents, separate from the phasic synaptic transmission
Extrasynaptic; alpha4, 5, 6
Synaptic; alpha 1,2,3

GABAA; pentameric ligand gated chloride channel - activation leads to influx of chloride. Sedation, anxiolysis, anticonvulsant and muscle relaxation

GABAB; heterodimeric Gi/o protein-coupled receptors which activate K+ channels and inhibit Ca2+ channels. Muscle spasticity (baclofen is GABAB agonist)

Question 4

GABA-A Receptor; Positive and Negative Allosteric Modulators

Describe the GABAAR in terms of ligand sites

Answer 4

3 major drug sites; 
GABA sit (agonist/ antagonist) in extracellular domain 

Picrotoxin site (channel blocker; NAM) in the transmembrane domain

BZD site (PAM) in the extracellular domain

High affinity ethanol site is located at extrasynaptic delta subunit GABAARs; modified BZD binding site which in turn is a modified GABA binding site

Question 5

GABA-A Receptor; Positive and Negative Allosteric Modulators

Describe BZs

Answer 5

Studies have demonstrated BZ drugs to act as CNS depressants and act as PAMs on GABAARs

BZ receptor binding is enhanced by GABA in the test tube

Alpha 4 and 6 GABAAR subtypes partner with delta subunits producing extrasynaptic GABAARs that mediate tonic inhibitory currents in the major cell populations including cerebellar and hippocampal granule cells = not BZD sensitive

Question 6

Beyond classical benzodiazepines: Novel therapeutic potential of GABAA receptor subtypes

Describe the role of alpha 1 beta gamma 2 receptors

Answer 6

Sedation 
Addiction 
Anterograde amnesia
Anticonvulsant activity 
Premature cortical plasticity

Question 7

Beyond classical benzodiazepines: Novel therapeutic potential of GABAA receptor subtypes

Describe the role of alpha 2 beta gamma 2 receptors

Answer 7

Anxiolysis
Anti- hyperalgesia
Antidepressant 
Cognition in SZ
Myorelaxation

Question 8

Beyond classical benzodiazepines: Novel therapeutic potential of GABAA receptor subtypes

Describe the role of alpha 3 beta gamma 2

Answer 8

Sensorimotor gating
Decrease in thalamic oscillations
Myorelaxation
Antihyperalgesia

Question 9

Beyond classical benzodiazepines: Novel therapeutic potential of GABAA receptor subtypes

Describe the role of alpha 5 beta gamma 2

Answer 9

Sensorimotor gating
Cognitive impairment
Myorelaxation

Question 10

Beyond classical benzodiazepines: Novel therapeutic potential of GABAA receptor subtypes

Abstract

Answer 10

GABAA receptors are a family of ligand gated ion channels which are essential for the regulation of the CNS function

BZD’s target GABAA receptors containing the alpha 1, 2, 3 or 5 subunits non-selectively - most widely prescribed drug for insomnia and anxiety disorders

Identification of key functions related to specific subunits of BZDs suggests that selective compounds could overcome the limitations of classical BZDs and may be valuable for indications such as analgesia, depression, SZ, cognitive enhancement and stroke

Question 11

Beyond classical benzodiazepines: Novel therapeutic potential of GABAA receptor subtypes

How do BZDs mediate their action?

Answer 11

Via modulatory binding site (BZD site)
on GABAA receptors

In contrast to barbiturates, the BZD binding effect is self limiting; the conductance of the channel in the presence of GABA and BZD is not higher than the conductance that can be achieved with high conc of GABA alone.

Additionally, in contrast to barbiturates, BZDs do not open the chloride channel in the absence of GABA

GABA induced chloride influx hyperpolarizes the post-synaptic neuron

Many GABAA receptors contain two alpha subunits, two beta subunits and one gamma with 2 GABA binding sites formed by alpha and beta subunits

The binding site for BZD is formed by the alpha and gamma subunit

GABAA receptors containing alpha 4 or 6 = BZD insensitive

Question 12

Beyond classical benzodiazepines: Novel therapeutic potential of GABAA receptor subtypes

Describe the role of GABAA receptor in pharmacological terms

Answer 12

BZD
Hypnotic drugs; zopiclone
Barbiturates
GA; etomidate and Propofol

Beta 3 k/o mice are unable to be immobilized by etomidate or Propofol, suggesting the role of beta 3 in immobilization

Volatile anaesthetics such as isoflurane, enflurane and sevoflurane act at a variety of targets, GABAA one of which

Question 13

Beyond classical benzodiazepines: Novel therapeutic potential of GABAA receptor subtypes

Describe GABAAR mediated events on the post-synaptic membrane

Answer 13

Increase in postsynaptic membrane conductance (shunting inhibition)

A change in the membrane potential due to movement of chloride ions through the membrane (hyperpolarizing inhibition)

Synaptic receptors that detect millimolar concentrations of GABA mediate fast inhibitory post synaptic potentials (IPSPs) whilst extrasynaptic receptors which detect micromolar concentrations of GABA mediate slower IPSPs alongside tonic conductance

Tonic and phasic conductance’s underlie different physiological and behavioural processes

Question 14

Beyond classical benzodiazepines: Novel therapeutic potential of GABAA receptor subtypes

Describe the role of human mutations in GABAAR subunits

Answer 14

Point mutations in alpha 1 and gamma 2; genetic epilepsy

SNPs in gene encoding alpha 2; alcohol dependence, illicit drug dependence
Beta 1 subunit of GABAAR has been linked to alcohol dependence and BAD

Genes encoding alpha 1, 6 beta 2 and pi subunits have been linked to SZ

Question 15

Beyond classical benzodiazepines: Novel therapeutic potential of GABAA receptor subtypes

How is the function of individual subunits of GABAAR identified?

Answer 15

Genetically modified mice in which GABAAR alpha subunits are rendered insensitive to diazepam via histidine to arginine point mutation - abolish binding of diazepam whilst conserving GABA nT action

Alpha 1 (H101R) mice: sedative, anticonvulsant action and amnesic action were reduced however anxiolytic action still present

Alpha 2 (H101R); anxiolytic like action and myorelaxant action missing whilst sedative action preserved

Tolerance = alpha 5 
Addictive = alpha 1

Question 16

Beyond classical benzodiazepines: Novel therapeutic potential of GABAA receptor subtypes

Describe binding and functional selectivity

Answer 16

Ligands at BZD site of GABAAR are allosteric modulators; e.g. they modify the efficacy and/or affinity of agonists e.g. GABA and thus regulate their activity

They can either by PAMs or NAMs depending on the stabilization of different confirmations of the receptor

Prototypic ligands for PAM, NAM and BZ site antagonist;
PAM = diazepam
NAM = beta-CCM
BZ site antagonist = flumazenil

Selectivity of a ligand for a specific GABAA receptor subtype can be obtained by binding i.e. by forming a receptor-ligand complex, or efficacy by eliciting a biological response after binding to the receptor
Binding and efficacy determine a potency profile

Question 17

Beyond classical benzodiazepines: Novel therapeutic potential of GABAA receptor subtypes

Describe the role of the GABAA site in addiction

Answer 17

All addictive drugs act to increase dopamine levels within the mesolimbic dopamine system
BZDs increase the firing of f DA neurons within the VTA by decreasing the activity of GABAergic interneurons

DA neurons in VTA express alpha 3 GABAARs; in contrast; GABAergic interneurons in the VTA express alpha 1 containing GABAA receptors which results in the disinhibition of DA neurons
Unselective BZDs modulate alpha 3 GABAARs on DA neurons within the VTA, but the disinhibition via alpha 1 containing GABAARs on interneurons is the predominant effect
This disinhibition triggers drug-evoked synaptic plasticity in excitatory glutamergic afferents onto DA neurons in the VTA and underlies drug reinforcements. In mice, a single dose of BZDs has been shown to elicit such neuroplastic changes

Alpha 1(H101R) mice showed no preference for midazolam indicating that alpha 1 containing GABAA receptors are essential for midazolam self-administration

Question 18

Beyond classical benzodiazepines: Novel therapeutic potential of GABAA receptor subtypes

Describe the role of BZDs in analgesia

Answer 18

Intrathecal BZDs act on central GABAARs such as those in the PAG - an area involved with the regulation of descending antinociceptive tracts; are pro-nociceptive at supraspinal sites.
In contrast, GABAARs in the spinal cord have anti-hyperalgesic actions

Alpha 2(H101R) and alpha 3(H101R) mice; intrathecal diazepam has a significantly reduced in models of inflammatory pain and neuropathic pain = spinal alpha 2 and alpha 3 containing GABARs mediate anti-hyperalgesic actions of intrathecal diazepam

Question 19

Beyond classical benzodiazepines: Novel therapeutic potential of GABAA receptor subtypes

Describe the role of BZDs on SZ

Answer 19

Alpha 3 knockout mice and alpha 5 partial k/o mice display deficits in sensorimotor fating, as determined by prepulse inhibition of acoustic startle, supporting a potential involvement of alpha 3 and alpha 5 containing GABAA receptors

Recent evidence suggests a role of alpha2/3 PAMs on cognitive impairment in SZ
In the cortex and hippocampus, hypofunction of NMDA receptors on GABAergic interneurons, which form synapses with the axon initial segment (AIS) of cortical pyramidal neurons to result in decreased inhibition of glutamergic pyramidal neurons. PM SZ brains show an upregulation of alpha 2 subunit in the AIS; compensatory adaptation